Throngs of well-wishers reportedly stood outside the palace as the Obamas arrived for the
late-afternoon meeting. Obama was the only head of state at the annual G-20 summit to get a private meeting with the queen.

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Later at a reception for all the visiting dignitaries," the queen put her hand on the back
of Mrs. Obama, who did the same for a few moments as they chatted," according to the British Press Association pool report, which called the gesture "an unusual step" in the tightly scripted world that surrounds a meeting with a British royal.

Meeting Queen Elizabeth -- even for a world leader -- can be nerve-racking. There is a long list of protocols that guides one's behavior in the presence of Her Majesty and even though the president and the first lady are not required to abide by all of them, there are certain formalities they do have to follow.

There is the "no-touch" rule, for example. The queen's visitors have to wait until she extends her hand to take it, and they are not supposed to grip it tightly or pump it, said Rachel Kelly, a public relations executive at VisitBritain, the U.K.'s official tourism office. No hugs, no kiss on the cheek, no touching the shoulder, which is why the queen's gesture comes as a surprise.

In 1992, Australian Prime Minister Paul Keating was assailed by the media when he put his arm around the queen. It's generally not considered socially acceptable to touch Her Majesty in any kind of way or to take her elbow to direct her.

The Obamas seemed to follow that rule well as they were greeted by Queen Elizabeth II and her husband, the Duke of Edinburgh, at Buckingham Palace. Even while posing for a picture, the four stood apart with hands in the front or back.

From Greetings to Gifts

Michelle Obama did not have to curtsy in the presence of the queen. Normally, British women do a small curtsy and men bow their heads when meeting the queen. Since they are not subjects of the royal family, the Obamas were not expected to comply, but President Obama did bow slightly from the waist as he met the queen and her husband.

But the key question is how does one address the fourth longest-reigning British monarch?

Kelly said one should refer to the queen as "Your Majesty" on the first reference, and then "ma'am" subsequently. The formal greeting for a U.S. president is simply "Mr. President."

Gift-giving is also an important part of the visit. The president and the first lady gave the queen a video iPod with an inscription, uploaded songs and accessories, plus a rare musical songbook signed by Richard Rodgers. According to one source, the iPod also contains video footage of the queen's state visit to the United States in May 2007, Obama's inauguration address and photos. The queen gave the president a silver framed photograph of herself and her husband, the official picture she gives all visiting dignitaries.

It remains to be seen how the public and media will react to the gift. Obama's gift to Prime Minister Gordon Brown -- a set of 25 American movie DVDs -- was assailed by the British press.